The present invention relates to an electrostatic capacitance detection device that reads the surface contours of a target object having extremely small projections and recesses, such as a fingerprint, by detecting electrostatic capacitance, which changes according to the distance from the surface of the target object, and also relates to a smart card including the electrostatic capacitance detection device.
In related-art electrostatic capacitance detection devices used for fingerprint sensors or other applications, a sensor electrode and a dielectric film provided on the sensor electrode are formed on a single-crystal silicon substrate. Examples of the related art are Japanese Unexamined Patent Publications No. 11-118415, No. 2000-346608, No. 2001-56204, and No. 2001-133213. FIG. 10 in this specification illustrates the operational principle of a related-art electrostatic capacitance detection device. A sensor electrode and a dielectric film serve as one electrode and a dielectric film of a capacitor. A human body serves as the other electrode grounded. The electrostatic capacitance CF of this capacitor changes according to the ridges and valleys in a fingerprint in contact with the surface of the dielectric film. Furthermore, another capacitor with electrostatic capacitance CS is provided for the semiconductor substrate. These two capacitors are coupled to each other in series, and a predetermined voltage is applied thereto. This voltage application generates a charge Q corresponding to the ridges and valleys in the fingerprint between two capacitors. This charge Q is sensed using typical semiconductor technologies to read the surface contours of the target object.
In recent years, it has been strongly suggested that personal identification functions ought to be provided on cards such as credit cards and bankcards to increase card security. Under this trend, fingerprint sensors detecting electrostatic capacitance have been expected to be used for a smart cart or the like since the fingerprint sensors can be fabricated as thin film devices and thus have smaller size and lighter weight compared with other biological identification devices. However, the related-art electrostatic capacitance detection devices, fabricated on a single-crystal silicon substrate, have little flexibility, which problematically causes difficulties of fabricating the detection devices on a plastic substrate.
To avoid this problem, a transfer technology referred to as SUFTLA (refer to Japanese Unexamined Patent Publication No. 11-312811, and Society for Information Display, p. 916 (2000), S. Utsunomiya et. al., for example) can be used to fabricate semiconductor integrated circuits on a plastic substrate. Using this technology eliminates the need to use an expensive substrate such as a single-crystal silicon substrate, produced with tremendous energy consumption, and thus allows the device to be fabricated at low costs without wasting precious global resources.
For the above-described technical background, it is preferable that electrostatic capacitance detection devices provided on a plastic substrate or the like are fabricated using thin-film semiconductor devices. However, it is very difficult, with current thin-film semiconductor device techniques, to fabricate electrostatic capacitance detection devices based on the conventional operational principle of FIG. 10 with using thin-film semiconductor devices. This is because the charge Q induced between two capacitors coupled in series is extremely small. Specifically, in contrast to single-crystal silicon LSI techniques that allow highly precise sensing and can therefore read the minute charge Q accurately, thin-film semiconductor devices cannot read the minute charge Q accurately since the thin-film semiconductor devices have inferior transistor characteristics and larger characteristics variation among devices compared with the single-crystal silicon LSI techniques.